In the past, the HVOF (hypersonic velocity oxy-fuel) continuous spraying of higher melting point powdered materials such as tungsten carbide (in a cobalt matrix) has required the use of oxidizers of much higher oxygen content than that contained in air. For example, in my earlier U.S. Pat. Nos. 4,416,421; 4,634,611; and 4,836,447 in particular, show forms of flame spray devices described as primarily oxy-fuel burners. Air may be one component of the oxidizer flow, but in each case the intensity of the flame jet relies on oxygen percentages greater than that contained in ordinary compressed air. The use of air to cool heated burner parts with this air subsequently entering and supporting the combustion process (regenerative cooling) was not feasible.
In place of "regenerative cooling", where the coolant becomes the oxidizing reactant, these prior flame spray devices rely on forced water cooling which severely limits the peak temperatures and jet velocities theoretically attainable. As an example, using a commercially available HVOF flame spray unit of the type discussed in U.S. Pat. No. 4,416,421, a simple heat balance shows that approximately 30% of heat released during the combustion process is carried away by the cooling water. Assuming a combustion peak flame temperature of 4,700 degrees Fahrenheit for a pure oxygen-propane mixture burning at a chamber pressure of 60 psig, if flame temperature was linearly related to heat content, then the 70% availability of the useful heat achieves a maximum flame temperature of only 3,150 degrees Fahrenheit. Of course, dissociation effects which limit the peak achievable temperature to 4,700 degrees F. release heat upon cooling. Thus, an actual combustion temperature of around 3,600 degrees F. is estimated.
Examining the combustion of compressed air and propane under conditions of essentially zero heat loss, the peak theoretical combustion temperature is about 3,400 degrees F. This is only 200 degrees F. less than that of the pure oxygen burner described above.